I. Field of the Invention
This invention relates to a surface light source device using an optical element (hereinafter referred to as "a light scattering guide") having a function which guides a light while scatting the incident light in a volume region, and relates to a liquid crystal display device using said device as a backlight source.
In more detail, the invention relates to a surface light source device which converts a light from an ordinary type of light supply source such as fluorescent lamp or the like into a uniform light flux having an enlarged sectional area for emitting.
The surface light source device according to the invention is advantageously utilized not only for an application to backlight source of the liquid crystal display device as mentioned above but also for the application of the light flux having a relatively larger sectional area, in particular, for the optional application requiring parallel light fluxes having the relatively larger sectional area, and therefore this may advantageously be utilized for various kinds of such devices with the types using the surface light source devices for the purposes such as illumination, floodlighting, information transferring etc.
II. Description of the Related Art
Conventionally, heretofore known is various kinds of optical elements or devices of the types capable of emitting the light to the desirous direction by utilizing scattering phenomena, and being used for such as the backlight sources of the liquid crystal display.
One typical example of these known techniques include, for example, Japanese Patent Application Laid Opens Nos. Hei-2-13925 in 1990 and Hei-2-245787, wherein a surface shaped light source is formed in that a light source is arranged on lateral-side of an extending plate shaped transparent material, a reflecting element is disposed on a back surface-side thereof, and the vicinity of front-side surface has a light dispersing property and made to a light emitting surface.
In the surface shaped light source using such technique, it is difficult in principle to sufficiently raise the rate of scattering light taken out from the light scattering guide device because there does not arise the light scattering in volume inside the transparent member and spread in the light emitting direction is only kept by utilizing a diffusive reflection or a mirror reflection which is in adjacent to the surface of the transparent member or in the reflecting elements.
In case where the surface light source having a uniform illuminance is intended by the light incident from lateralside, as is easily understood from the examples in the Laid Open Document as described, some gradient is required for a reflective power of the reflection elements, this requires a complicated and larger sized construction for the light scattering guide device portion, it is therefore impossible to avoid a higher production cost.
Another similar type of the known technique is such that particle shaped substance having different refractive index from the transparent material is diffused into an extended plate shaped transparent member. This technique is considered to be superior to the first similar type of technique as described in a view point that a region having a volume spread has a scattering power to provide a directional conversion of the incident light to a direction of a light output surface. However, a problem still remains in that a particular countermeasure is needed in order to secure a uniformity of the brightness as a surface light source.
As an example, in Japanese Patent Application Laid opens Hei-2-221924, Hei-2-221925, and Hei-2-221926 each in 1990, a means is provided for enlarging a particle diameter and/or a particle concentration depending on an increase of distance from the light incident surface. However, it is difficult in technique to actually manufacture the light scattering guide having a gradient on the particle size or particle concentration. Such light scattering guide has disadvantageous and less productivity. For example, it is difficult to obtain a light scattering guide by one-time molding by applying an injection molding technique.
The present inventor has proposed a surface light source device having a uniformity of illuminance which is obtained using a light scattering guide with an tendency to reduce its thickness with an increase of distance from the light incident surface (Japanese Patent Application Hei-5-201990 in 1993). Such proposition is shown in FIG. 1 as one example of a basical structure of the surface light source device therealong.
For simplified explanation therefor, numeral 1 depicts a light scattering guide having a wedge shape, in adjacent of its light incident surface 2 is arranged a light source (a fluorescent lamp) L. A light emitting direction correcting element 4 having a line shaped prism surfaces 4a and 4b (its operation is described later) is disposed on a light output surface 5 side of the light scattering guide 1. A light flux 4f with a directionality having its flat surface 4e as a bright section is obtained. Arranging on upper portion thereof the known liquid crystal display device (called "a liquid crystal panel" depending on circumstances) composed of liquid crystal cell and other elements, thus a liquid crystal display is constituted.
Numeral 3 depicts a reflection means which is disposed opposing to a back surface 6 of the light scattering guide 1 and is made of a white sheet with a diffusive reflection property or a silver foil sheet with a regular reflection property.
In such surface light source device, a thickness of the light scattering guide 1 is more thinned with the increase of distance from the light incident surface 2 side, thus, due to a sloped surface repeating reflection effect which is arisen within the light scattering guide 1, a brightness level and a uniformity degree as a surface light source are considerably improved. A reason of obtaining such result is described as follows referring FIG. 2 which is a sectional view of a wedge shaped light scattering guide 1 and a reflector 3 used in an arrangement in FIG. 1.
Assuming that B0 represents a ray taken into the light scattering guide 1 from the light incident surface 2, then B0 is considered to form a small angle with a horizontal direction as shown in the drawing.
The behavior of this ray B0 is as follows. The ray B0, while receiving a change of direction due to scattering at a specified rate, repeats reflection on the light output surface 5 and the sloped surface 6 as shown in the drawing, and comes to the thin thickness portion of the light scattering guide 1. Reflections on the surfaces 5 and 6 are regular reflections, and an incident angle and a reflection angle on each reflection is as a matter of course equal to each other (.theta.1, .theta.2, .theta.3 . . . ), where, on attention to reflection on the light output surface 5, a relationship of .theta.2&gt;.theta.4&gt;.theta.6 is established.
In consideration of a boundary face transmittance on each reflection, a total reflection arises under the condition of .theta.i&gt;.alpha.c (a critical angle; 42.degree. at PMMA--air boundary face), a transmittance rapidly rises on lowering of .theta.i less than .alpha.c, and a transmittance comes substantially constant on .theta.i equal to or less than a predetermined value (around 35.degree. at PMMA--air boundary face). In the drawing, a situation of producing the emitting lights B4 and B6 by a relationship of .theta.2&gt;.theta.c&gt;.theta.4&gt;.theta.6 is shown.
Such effect is not limited to the representative ray B0 (non-scattering light), and the effect must similarly be produced as to a primary scattering light or a multi-scattering light, hence, as a whole of light scattering guide 1, it is considered to produce an effect that a light emitting rate from the light output surface 5 is more raised with the increase of a distance from the light incident surface 2. In evaluating this effect by a function f(x) of a distance "x" from the light incident surface 2, f(x) is an increase function relating to "x". On the other hand, in the vicinity of the light incident surface 2, an effect of being close to the light source 5 exerts on both of a direct light and a scattering light. This proximity effect is evaluated by g(x), then g(x) is a reduction function.
The proximity effect g(x) is canceled by f(x), thereby a tendency arising wherein a light is allowed to emit from the light output surface 5 after the light is introduced to a longer distance as much as possible. An opportunity, in which the light within the light scattering guide 1 is incident on the light output surface 5, is considered to increase as a whole by effect of the wedge shape, it can therefore be expected in effect that a brightness level itself is remarkably improved as a surface light source. In case of using one having a regular reflection property (silver foil sheet etc) as a reflector 3, a light transmitted through back surface of the light scattering guide i is not diffused and comes to more have a probability to be reincident into the light scattering guide 1, the effect as described above comes clearer accordingly.
In this way, by using a light scattering guide having an tendency to reduce a thickness with an increase of distance from the light incident surface, there may be constituted a surface light source device with an upgraded uniformity of the brightness and illuminance. However, even in such an improved type of surface light source device, a problem still remains in an tendency to generate a brightness irregularity of the surface light source mainly in an adjacent region to the light incident surface. According to consideration of the present inventor, the mechanism of producing such phenomenon is summarily explained referring to FIG. 3.
FIG. 3 shows an adjacent region of a light incident surface 2 in FIG. 2. A number of reflected rays having a smaller incident angle, such as being represented by rays C1 and C2, which travel forward and backward, are produced within the light scattering guide 1 in adjacent to the light incident surface 2 provided that a light source element L is not to emit only a parallel light flux vertical to the light incident surface 2 in case of arranging the light source element L in adjacent to the light incident surface 2 as a light supply means for supplying to the light scattering guide 1.
For example, now considering a specified ray C1 which enters to the light scattering guide 1 from the light incident surface 2 at a smaller incident angle and incident on a position R1 of a back surface 6 of the light scattering guide 1 at an incident angle .phi.1 without being scattered, then a part thereof is reflected on R1 to be towards the light output surface 5 side. In addition, although not shown in the drawing, a part of the light which has not been reflected on R1 is reflected on the reflector 3 and re-incident within the light scattering guide 1, and travels toward the light output surface 5 side through the same optical path.
An incident angle .phi.2 of a light which is incident on a position R2 to the light output surface 5 from the light scattering guide 1, decreases less than .phi.1 to approximates to 0.degree. because of the sloped back surface 6 for the light output surface 5, and is largely reduced by an angle exceeding a critical angle (for example, approximately an extent of 42.degree. in case of using acrylic resin on the light scattering guide 1). Therefore, most of the light is emitted from the light output surface 5.
As seen in the ray C2, the scattering light which contacts on a scattering center within the light scattering guide 1 (a refractive index ununiformity structure), has a strong forward scattering property as described later, and thus possesses a high probability to be incident on the back surface 6 of the light scattering guide 1 at a small incident angle. For this reason, the ray C2 is also reflected, on its both surfaces 5 and 6 and on the reflector 3 in the ray C1, thereby lights, which are mixed of a number of strong and weak lights as shown by C12 in the drawing, are emitted and produce a local ununiformity zone on brightness of the light output surface 5. In particular, on usage of one having a regular reflection property for the reflector 3, it will be of a high possibility to appear such ununiformity more notably.
Such brightness irregularity should be referred to as "reflective appearance phenomenon of the light source L in accordance with a regular reflecting phenomenon", accordingly hereinafter referred to simply as "reflective appearance".
On occurrence of the brightness irregularity due to the reflective appearance, the light output surface having a uniform brightness is substantially reduced and it is apparent that an extremely adverse effect practically occurs. For example, for application to the backlight source of liquid crystal display, it is not possible to avoid a countermeasure for shielding the light on the portion on which the brightness irregularity arises, thus, the size of display surface is limited correspondingly. A lowering of a light utilization efficiency deteriorates an electrical power saving characteristic.
One characteristic which is desired for the surface light source device is a directionality of an illumination light. For example, the surface light source device used for the backlight source of the liquid crystal display often requires a clear directionality in addition to a largeness of a sectional view of the illuminating light flux, a level of brightness or illuminance, a uniformity and so forth. This is in many cases because, in the liquid crystal display, the brightness of the display is of importance as to an observing direction in a limited range of a certain extent. In such a case, it is disadvantageous to use as a backlight the light flux (a light flux without having a parallel property) which is propagated in an excess wide angular range.
However, it has been difficult that a light emitted from the ordinary light source such as a fluorescent lamp or the like is converted into a uniform flux having a larger sectional area. For the known means for converting the light flux having a non-parallel property into a light flux with a high parallel degree, the optical system formed of lenses or a curved face mirror (a parabolic mirror etc) is well known. This light flux parallelization technique using the lens or the curved face mirror is to parallelize a light emitted in multi-direction from a non-parallel property flux source (a single number or a plural number of light source element(s) or an optical fiber emitting end or the like) by means of functions of light convergence or divergence which the lens or curved face mirror possesses.
Therefore, when the non-parallel property light flux emitting source has solely either one property of the divergence property (for example, a single point light source) or the convergence property (for example, a good number of directional point light sources arranged forward one point), then this comes to an effective parallelization means. However, it is difficult to parallelize the light flux which is difficult to regulate by one side property of divergence or convergence in case of the emitted light in multiple-directions from the light source having a spread property.
In using the optical system composed of the lens or concaved mirror, it is difficult in principle to flatten a profile of the parallelized light flux (the distribution of light intensity in the light flux sectional view; in most cases, the Gausian type profile having a peak in the vicinity of the optical axis is given), therefore this is not suitable for the application to obtain the parallelized light flux having a uniform light intensity.
Furthermore, in order to cause the lens or the curved face mirror to exhibit the light convergence/divergence function, a sufficient distance along an optical axis direction is required to be secured, thus miniaturization in a depth direction as a whole device is difficult in general,
Heretofore known is the technique in that a light which is incident from lateral side by light scattering condition is emitted from the light output surface of the front surface-side as is the case of the surface light source devices described in Japanese Patent Application Laid Open Hei-4-145485 in 1990 and Japanese Utility Mode Registration Patent Application Laid Open Sho-51-89888 in 1978 in addition to the known documents (Japanese patent Application Laid Opens Hei-2-13925, Hei-2-245787, Hei-2-221924, Hei-2-221925, and Hei-2-221926 each in 1990).
However, those techniques are based on the conception such that a light amount emitted from the light output surface is secured through making a forward direction of the light to be random as much as possible by the light scattering function which is given to the inside of the light guide or to the surface region. Therefore, these known techniques do not intend to solve technical problems for obtaining the parallelized illuminating light from the surface light source device.
In this manner, conventionally, producing the parallelized light flux having a uniform profile, is in difficulty and consequently it is so difficult to secure a largeness of the light flux sectional area simultaneously in addition to the parallel degree of the light flux and uniformity of the profile.